Fuel Pressure Specs?

[73ess]

Anyone know what the correct fuel pressure should out of the fuel pump on a 73S? Then the fuel pressure after the fuel filter and on the return line from the MFI pump? I'm getting 14psi at the pump, 13 after the filter and 12 on the return line from the MFI pump.

[njhj1052]

Hi there

The correct pressure on the return line is 11.8 PSI, + or - 3 PSI so you are fine. Also the correct flow rate from this line is 900 to 1000 cc in 30 seconds. This is as important as the pressure and is well worth checking.

Taken from the Porsche factory workshop manual.

Cheers

Nigel

[edmayo]

On the pumps I have tested I got 35 psi when measuring directly on the pump output. As the post above mentions there is also the volume test. The 11-12 psi at the return line is correct.

[304065]

Hi there

The correct pressure on the return line is 11.8 PSI, + or - 3 PSI so you are fine. Also the correct flow rate from this line is 900 to 1000 cc in 30 seconds. This is as important as the pressure and is well worth checking.

Taken from the Porsche factory workshop manual.

Cheers

Nigel

Uhh, this is a 73S, so the working pressure in the loop between console and pump should be 1 bar, and the fuel pressure relief valve in the pump opens at 2 bar.

[73ess]

Thanks. So the pump output pressure should be closer to 35psi? So I guess my pump output pressure is way too low at 14psi? But the return line pressure from the MFI pump is within spec. With 14psi out of the pump and 12psi at the MFI pump return, should my car stay running assuming all else it fine?

[304065]

Thanks. So the pump output pressure should be closer to 35psi? So I guess my pump output pressure is way too low at 14psi? But the return line pressure from the MFI pump is within spec. With 14psi out of the pump and 12psi at the MFI pump return, should my car stay running assuming all else it fine?

No. 14 PSI working pressure is fine in the loop. Fuel pressure is important to MFI, but the step in Check Measure adjust is to make sure there's 1) actually flow happening and 2) sufficient volume flowing through it to eliminate vapor lock and keep the fuel gallery around the injection pump pistons filled. It's not like CIS or EFI where the fuel pressure determines injection quantity. 12 psi suggests a dying pump, how old is it?

35 psi is 2,4 bar and is too high, the relief valve should be open by then and sending excess fuel back to the tank until the pressure falls to 2 bar or lower.